I. Field of the Invention
This invention relates to improved 2-cyanoacrylate adhesive compositions having enhanced bond strength. More particularly, this invention is directed to an adhesion promoter for 2-cyanoacrylate adhesive compositions which does not significantly reduce the cure rate thereof.
II. Description of the Prior Art
Adhesive compositions based on 2-cyanoacrylate esters belong to a class of adhesives known as reactive liquid adhesives. 2-Cyanoacrylate adhesives are single-part, low-viscosity adhesives which are characterized by features such as (1) their ability to polymerize at room temperature without the use of an added catalyst when pressed between two substrates, (2) their rapid rate of cure, and (3) the strength of the bonds produced with a wide variety of substrates. Conventional adhesives, on the other hand, cure, for example, upon application of heat and pressure, addition of catalyst, or evaporation of a solvent. A general review of 2-cyanoacrylate adhesives can be found in I. Skeist's "Handbook of Adhesives", New York: Reinhold Publishing Corporation, 1962, Chapter 31, p. 409-414.
Application of the 2-cyanoacrylate adhesive merely involves spreading a small sample thereof in a thin film between two substrates, pressing the substrates together, and allowing the resultant bond to cure. The adhesive develops sufficient strength after a short period of time to hold the substrates together until the adhesive completely polymerizes and builds up to its maximum bonding strength.
Initiation of polymerization, or cure, is generally believed to proceed through an anionic mechanism; the 2-cyanoacrylate adhesives have such a great tendency to polymerize that water itself is a sufficiently active initiator. Hence, when the adhesive is applied to a substrate and thereby exposed to atmospheric and surface moisture, cure normally begins within a relatively short period of time, generally less than one minute, and on many surfaces within a matter of a few seconds. The rapid cure rate of the 2-cyanoacrylate adhesives is particularly advantageous in production line applications.
Due to their tendency to polymerize, the 2-cyanoacrylate adhesive compositions normally contain one or more stabilizers. To prevent anionic polymerization, an inhibitor such as an acidic gas or a protonic acid or anhydride thereof is normally added to the composition. In general, as the acidity of these inhibitors increases, the stability of the adhesive is enhanced while, at the same time, the cure rate is reduced. Free radical polymerization is generally inhibited in the adhesive composition, if necessary, by adding phenolic-type compounds such as hydroquinone or t-butyl catechol thereto. Typical patents which describe the use of these and other compounds as stabilizers for 2-cyanoacrylate adhesive compositions include U.S. Pat. Nos. 2,765,332 and 3,993,678; and Japanese Patent Publication No. 49-31619.
Although adhesive compositions consisting of 2-cyanoacrylate esters and conventional stabilizers inherently yeild high bond strength (as commonly measured by the test of tensile shear strength), improvements therein would be desirable particularly in cases where the substrate is of greater strength than the adhesive, as in the case of many metal bonds. While many stabilizers for 2-cyanoacrylate adhesive compositions have been investigated in the prior art, relatively little research has been done on adhesion promoters which enhance the bond strength of the adhesive without lessening the stability or cure rate thereof. In this regard, selected carboxylic acid anhydrides have been used in the prior art as adhesion promoters for 2-cyanoacrylate adhesive compositions. Thus U.S. Pat. No. 3,832,334 teaches the use of maleic anhydride to increase the bond strength of a 2-cyanoacrylate adhesive at elevated temperatures, while U.S. Pat. No. 3,948,794 teaches the use of itaconic anhydride to improve not only the stability of the 2-cyanoacrylate adhesive composition but also the tensile shear strength and other properties associated therewith.
It has been disclosed in German Offenlegungsschrift No. 26 12 546 that a wide variety of carboxylic acids, generally known to stabilize 2-cyanoacrylate adhesives to varying degrees, exert a definite effect on the bond strength of certain 2-cyanoacrylate adhesive compositions containing excess plasticizer. The plasticizer is added in amounts of 20-60% by weight, based on the total composition, to make the adhesive bonds readily releasable. It is suggested that one of the functions of the carboxylic acid therein is to counteract the weakening effect of the plasticizer on the bond strength of the adhesive. Nevertheless, the large number of carboxylic acids described as being applicable in the German publication limits the use of the adhesive composition to very few types of substrates. For example, many of the carboxylic acids listed therein, i.e., those with a high acid strength (low pK.sub.a), act very effectively as anionic polymerization inhibitors and hence will significantly retard the rate of cure of the adhesive, particularly on non-polar substrates, within the concentration range given therein. Yet, as described hereinabove, the rapid rate of cure is one of the main characterizing features of 2 -cyanoacrylate adhesives in general. Furthermore, it has been demonstrated that at least some of the weaker acids (with relatively high pK.sub.a) within the scope of the German publication, which acids do not significantly reduce the rate of cure of the adhesive composition, have in fact very minimal effect on the bond strength of the composition on many metals.
Accordingly, it is an object of the present invention to provide an improved 2-cyanoacrylate adhesive having enhanced bond strength.
It is another object to provide an adhesion promoter for 2-cyanoacrylate adhesive compositions which does not significantly reduce the cure rate thereof.
It is a further object to provide a process for improving the adhesive properties of a fully stabilized 2-cyanoacrylate adhesive composition.